1. Field of the Invention
The present invention relates to a canine femoral stem for canine hip replacements; and more particularly to a femoral stem construction that achieves a firm bond between the femoral stem and the underlying reamed bone canal, thereby providing initial stability immediately following surgery, together with long-term stability.
2. Description of the Prior Art
Many patents address issues related to hip replacement, and especially to femoral stem replacement. These patents disclose methods to attach the femoral stem into a reamed bone cavity using cements having various formulations, or screws and barbs associated with cementless procedures. Several patents address primarily human hip replacement while a few of them relate to canine hip replacement. There are large significant differences between a femoral stem designed for human hip replacement and one appointed for canine hip replacement. These differences are in part due to the larger overall weight of the human body as compared to that of a dog. Human devices require femoral stems having larger diameter and lengths, and additional attachments heretofore considered unnecessary for canine use. These dimensions are severely restricted when femoral stems are used in canines. The design of femoral stem must provide initial stability immediately after surgery and restrict the movement of the implanted femoral stem so as to extend the in-service life of the attachment between the femoral stem and the bone canal. Thus, the geometries of femoral stems appointed for canine use are required to be different from those used in humans.
U.S. Pat. No. 4,031,571 to Heimke et al. discloses a hip endoprosthesis having stepped load-transferring surfaces. The thigh portion of an endoprosthesis for hip joints is provided with stepped projecting surfaces to distribute forces in the bone tissue. Projecting surfaces provide a larger area of contact with the bone tissue, reducing stress intensity. These steps of projections are provided at opposing angles in the proximal end to provide initial stability of insertion of the animal's femoral stem. The projections are larger in size and insertion of the implant into bone cavity results in gaps between the implant and the bone, contrary to what is shown in the figure. This lack of contact between the steps will prevent bone filling and does not provide increased contact area as claimed in the patent. Due to the large size of these projections, this device is obviously intended for human use; and would not be suitable for use in canines.
U.S. Pat. No. 4,407,022 to Heimke et al. discloses a femur component for an artificial hip joint. Initial stability of the animal femoral stem is achieved by use of steps that are inclined to the longitudinal axis of the femoral implant step. One set of steps faces upward and a second set faces downwards. Fitting this stepped stem into a bone cavity pushes the larger size into the bone cavity creating a larger hole. This prevents bone contact with the portions of the implant that do not carry the stepped regions, creating a poor bond between the femoral stem and the bone. Also, the surface of the femoral stem is not coated with a porous coating for bone ingrowth. Rather, the device disclosed by the '022 patent relies on the steps to provide initial and long term implant stability. Due to the large size of these projections, this device is clearly intended for human use; it would not be suitable for use in canines.
U.S. Pat. No. 4,546,501 to Gustilo et al discloses a hip prosthesis. A femoral insert combines bone ingrowth surfaces in the proximal portion of the shaft, which is wide in the medial-lateral direction and curved in the medial aspect with a distal portion, which has a round cross-section and is curved in the direction of the anterior bow of the femur. Accordingly, firm fixation of the implant is initially provided by the distal portion functioning as an intramedullary rod, while long-term stabilization is achieved proximally through bone ingrowth. Also, the neck connecting the spherical head to the shaft through the collar is anteverted with respect to the collar at an angle of about 10 degrees to 12 degrees. The prosthesis for the left femur is basically a mirror image of the prosthesis for the right femur. The '501 patent discloses a hip prosthesis for humans based on the size of the femoral head, which is 28 mm. Initial stabilization is accomplished by a curved fit between the distal end of the femoral stem and the bone cavity. A collar resting on the bone's outer surface provides additional rigidity. There is no real support provided at the proximal end. A porous bone bonding coating is only provided on the sides of the rectangular cross section implant device; it does not completely surround the proximal region of the implant. The device disclosed by the '501 patent would not be suitable for canine use.
U.S. Pat. No. 4,589,883 to Kenna discloses a femoral hip prosthesis. A femoral hip prosthesis comprises a stem component generally divided into a proximal portion and a substantially longer distal portion. The stem has a slight posterior bow along its length. The proximal portion includes a slight twist of from about 5 degrees to 15 degrees. The twist extends in a direction from the anterior to the posterior face of the stem through the medial face thereof. Stem characteristics provide a glove fit when the device is inserted into a prepared intermedullary canal in the femur. A curvature in the mid portion of the femoral stem binds the implant against the bone cavity, providing initial stability. No support is provided elsewhere. The proximal portion of the femoral stem is coated with bone ingrowth coating. Owing to the large angle of twist (i.e. 5-15 degrees), the femoral stem disclosed by the '883 patent is appointed for use in humans; but not canines.
U.S. Pat. No. 4,718,912 to Crowninshield discloses a femoral component and method of constructing the same. A femoral component for a hip prosthesis includes a stem with a porous surface or the like to enhance bone fixation. The porous surface is applied to the stem in a predetermined manner to accommodate stress lines for the stem and generate a curvilinear inferior boundary for the porous surface. It uses a porous coating that encourages bone ingrowth at locations where stress is low, accommodating the differences in stress experienced by the anterior side as compared to the posterior side of the stem. The femoral implant device is said to be useful in human; its suitability for canine use is not disclosed. There is no provision for providing initial stability of the femoral stem, since stability is based on bone ingrowth in the porous region, which is a long-term stability issue.
U.S. Pat. No. 5,047,056 to Kenna discloses a canine hip prosthesis with a porous coating for bone ingrowth. The stem for insertion into the intramedullary canal of a canine femur is divided into a proximal portion and a longer distal portion. A compound curvature along its length provides a glove-like fit in the prepared intramedullary canal of the canine femur. The proximal portion of the stem includes a porous coating of two layers of ball-shaped metallic particles bonded together at their points of contact. Pore size of the coating ranges from about 0.014 to 0.020 inches. The stem of the canine hip prosthesis is held in place in the bone cavity primarily by the posterior bow. Further curvature in the proximal portion and the overall three dimensional curvature create a glove-like fit. The initial stability is entirely provided by this curvature. No support is provided either at the distal or proximal end. This curvature does not result in intimate contact with the bone cavity. Porous coating applied at the proximal end and bone ingrowth is hindered due to micro motions at this proximal end.
U.S. Pat. No. 5,047,062 to Pappas et al discloses a femoral stem-type prosthesis. The bending stress in the neck of the femoral stem is reduced by limiting the length of the neck and positioning the femoral head nearly in line with the right edge of the collar. There is no disclosure on how the stem is attached or how initial stability is attained. Neither is there any disclosure in the patent that the femoral stem could be used for canine applications.
U.S. Pat. No. 5,108,452 to Fallin discloses a modular hip prosthesis. A modular hip prosthesis can be custom fitted to a particular patient by a surgeon prior to surgical insertion. The prosthesis features a body having a neck portion for carrying a rounded head element, a transitional mid-section of the prosthesis body includes generally rectangular and generally rounded cross-sectional areas. A stem section of the prosthesis has a generally rounded cross-sectional area. The stem is tapered to receive a tubular extension sleeve with a hollowed portion corresponding in shape to the stem portion of the prosthesis. The tubular extension sleeve has an open-end portion receptive of the lower tapering stem of the prosthesis body. The stem portion includes an internal bore. An attachment in the form of an elongated screw is provided for connection to the stem internal bore. By this means the extension sleeve and the prosthesis body are secured together, forming a compressive sealed connection therebetween. The modular hip prosthesis is appointed for human use, not canine use. It employs a number of femoral stem components that work collectively to form an integral custom shaped femoral stem. The stem itself is made in several pieces. A single bolt, when tightened, bends the cylindrical distal end at a slight angle complying with the bone cavity. The presence of bolts in the system holding together a geometry subjected to loading creates problems. Motion of the bolt and components of the device, or slight deformation of the bone cavity due to load, causes shifting of the device. There is no porous coating for bone attachment. The cylindrical distal end does not provide firm attachment. Moreover, in the device disclosed by the '452 patent, placement of the implant relies on the overall bending of the cylindrical extension. The initial and long-term stability of this patented device is therefore questionable. Various femoral heads are provided; but they are attached through a collar providing long-term reliability problems. The device taught by the '452 patent is not suitable for canine use.
U.S. Pat. No. 5,116,379 to McLardy-Smith discloses a prosthesis. A hole in the proximal section and the shaft of the prosthesis has a diameter over 12 mm. As such, the prosthesis disclosed by the patent is intended strictly for human use. The distal section and proximal section are separately formed and held together by the shaft. Generally, the proximal section is wedge fitted into the bone cavity. Alternatively, a smaller proximal section may be cemented within the bone cavity. There always exists a play between the shaft and the hole, so that the proximal section may be slid over the shaft. This very play causes initial and long-term instability problems. There is no coating to create bone ingrowth. The device disclosed by the '379 patent would not be suitable for canine use.
U.S. Pat. No. 5,336,265 to Serbousek et al. discloses a reduced stiffness femoral hip implant. A femoral component of an artificial hip joint includes an elongated stem with a single longitudinally extending, medially facing, groove in its outer surface intermediate its proximal and distal ends. The dimensions of the groove are chosen so as to obtain constant stiffness or, alternatively, a predetermined stiffness of the stem between the proximal and distal ends. In either event, the stem has a magnitude of stiffness no greater than that at which stress shielding of the first bone would occur. The groove and the outer surface of the stem intersect at an outer rim. The component is preferably composed of titanium, titanium alloy, or cobalt-chromium alloy. The outer surface of the stem may be provided with a porous medium for encouraging bone ingrowth fixation. This reduced stiffness femoral hip implant is said to be appointed for human use. There is no initial stability in this device. Long-term stability is provided by the bone ingrowth coating if the movement of the implant is restricted. However this restriction is not readily accomplished by the design of the reduced stiffness femoral hip implant. The device disclosed by the '265 patent would not be suitable for canine use.
U.S. Pat. No. 5,456,717 to Zweymuller et al. discloses a stem for a femoral hip-joint endoprosthesis. The stem for the femoral hip-joint endoprosthesis comprises a shaft, which is substantially rectangular in cross section, with a distal and a proximal end wherein the distal end tapers substantially conically and the proximal end (16) comprises an insertion means and an extraction means for the insertion and for the extraction of the stem into a narrow space within a femur. A cone is provided to receive a joint ball. The cone is connected to the proximal end of the shaft by way of a neck. The disclosure does not state that the femoral hip joint is for canine use. It would be very difficult to create a rectangular bone cavity in a dog to receive this rectangular stem femoral stem. In addition, there is no provision for initial stability or presence of a porous coating for long-term stability due to bone ingrowth.
U.S. Pat. No. 5,507,833 to Bohn discloses a hip replacement system and method for implanting the same. A femoral stem has a series of dimples provided at the distal and proximal ends of the femoral stem, which may be coated with hydroxyapatite to create bone bonding. This is a device for human use based on the length of the femur stem (approximately 100 mm to 180 mm). There is no initial stability since both distal and proximal ends of the femoral stem await bone growth.
U.S. Pat. No. 5,725,594 to McTighe et al. discloses a proximal conical stem. A cementless femoral hip stem component includes an elongate stem having opposing distal and proximal sections. The proximal section includes a conical stem surface, which terminates in a proximal end. A collar having a conical undersurface extends laterally outward from the proximal end of the stem. The proximal conical stem portion and the conical undersurface of the collar cooperatively define a unitary double-cone contact surface to allow the collar to subsidably engage with external cortical bone in tandem with the proximal conical stem subsidably engaging with the internal femoral canal. The proximal portion of the femoral stem is made up of two opposing conical and frusto-conical sections, thereby making contact with their corresponding bones to provide improved load capacity and improved bone ingrowth. This is a device made for human use; no disclosure is contained by the patent concerning canine use.
U.S. Pat. No. 5,725,595 to Gustilo discloses a cannulated cementless hip stem prosthesis. A canulae feature of the stem component permits the stem to be precisely aligned in an intramedullary canal which is prepared in a person. The distal end is flexible while the proximal end contains bone ingrowth stimulating bead coating. This is a device for human use and not for canine use.
U.S. Pat. No. 5,755,805 to Whiteside discloses a tapered prosthesis component to be used primarily in humans. The tapered prosthesis component is rectangular in shape with ridges and steps cut into the tapered portion to anchor the component into a bone cavity. The wedging action and cutting action of the ridges is set to anchor the component into the bone. If the tapered bone cavity is not prepared correctly, the femur bone may fracture. There is no porous coating on the implant to encourage bone growth. Producing this matching tapered bone cavity by reaming would be difficult in canines and thus is not suitable for canine use.
U.S. Pat. No. 5,871,549 to Jayashankar et al. discloses a femoral stem with reduced coefficient of friction with respect to bone cement. This cemented femoral stem has a lubricious coating between the femoral stem and the bone cement to reduce damage caused by bone cement and the creation of wear particles. The reduced coefficient of friction also allows movement of the femoral stem in the bone cavity with the bone cement, compromising the initial stability of the femur implant. This is not a femoral implant for canine use.
U.S. Pat. No. 6,102,957 to Noble et al. discloses an asymmetric hip stem for use in cementless hip replacement procedures. The stem comprises a proximal region having a novel three-dimensional configuration that allows for better fit and stability of the stem within the femoral intramedullary canal, with minimal removal of strong bone therefrom. The stem further includes a twist isolated in the mid-stem region to improve fit while minimizing enlargement of the femoral canal. A distal end has a rotated internal slot, which reduces bending stiffness of the stem in both the coronal and sagittal planes. This asymmetric hip stem is for human use. Its length is preferably about 4.5 inches to 6.5 inches (col. 5, lines 41-43). Its femoral neck is relatively long, being about 18 mm (col. 5, lines 13-17). Complex asymmetric geometry is used to improve fit and stress distribution. The device disclosed by the patent is not suitable for use in canines.
U.S. Pat. No. 6,413,280 to Feiler discloses hip joint prosthesis. The patent discloses hip joint prosthesis for humans which provides a larger degree of hyperflexion of the leg by designing a neck which has a complex shape. The acetabular cup is notched to prevent interference of the neck during hyperflexion. This femoral stem is not for canine use.
U.S. Pat. No. 6,428,578 to White discloses a modular prosthesis and connector therefor. Three components: a stem, a sleeve and a body, are assembled during surgery at the proper height to create the femoral stem. The body has a bore into which a sleeve and then the stem is inserted. Force is applied to lock the stem within the sleeve. A single unit meets different size needs. No disclosure is contained by the patent concerning a system for selection of differently sized femoral stems. Moreover the device disclosed by the patent is intended for use in humans, not for canine use.
U.S. Pat. No. 6,464,728 to Murray discloses a modular neck for femur replacement surgery. The modular neck is attached into a blind hole in the stem at selected anteversion to meet patient requirements. The device disclosed by the patent is designed for human use and is not for canine use.
U.S. patent application No. 2002/0058999 to Dwyer et al. discloses a modular prosthesis having a stem component with a counterbored cavity defined therein and an associated method. The modular prosthesis is disclosed as being suitable for human use; not for canine use. It uses a common and proximal member but the length of a distal member may be changed according to the femur length of a patient. All attachment means are tapered and held by threaded connection. It is not an integral stem and may be subject to loosening resulting in the displacement of the implant. It does not have porous coating to facilitate bone ingrowth.
U.S. patent application No. 2002/0138151 to Hubbard et al. discloses a cementless femoral hip stem component. The hip stem component includes an elongate stem, a proximal body, and a collar disposed on the proximal body and extending outward therefrom in a sideways direction to form a ledge. The proximal body defines at least a majority of a frustoconical shape. The ledge formed by the collar includes a tapered undersurface, beneath which extends the proximal body. The tapered undersurface of the ledge and the proximal body extend in different directions and thereby cooperatively define a double-flared contact surface configured to enable subsidable engagement of said double-flared contact surface with the femur at two different rates of subsidence within the femur. An abrupt, male corner is formed by the stem and the proximal body and is sufficiently abrupt to provide enhanced bone ingrowth stimulation. The prosthetic hip replacement system uses different porosity coatings at various locations of the stem and adjusts the location and angle of the neck element to minimize the stresses and maintain a natural biomechanics, which is clearly for human use. No disclosure is contained by the patent application concerning a canine femur stem.
There remains a need in the art for interchangeable canine femoral stems selected from a set of sizes permitting use of cemented or cementless procedures based on the situations during surgery. The implanted canine femoral stem must be usable immediately after surgery and develop long term permanent bond between the implant and the underlying bone structure meeting the needs of a dog. The implanted device must function satisfactorily throughout the lifetime of the dog without damage or deterioration.